This invention relates to waterborne compositions for staining porous substrates such as wood.
Reduction of the amounts of organic solvents in coating compositions has been a desirable objective for the sake of reducing the amount of volatile organics released into the atmosphere. Therefore, waterborne coating compositions have come into widespread use. Waterborne stains have also been proposed (e.g., U.S. Pat. Nos. 4,432,797; 4,976,782 and 5,149,729), but providing acceptable performance with a waterborne stain has generally been hampered by the tendency of water to raise the grain of the wood to which a waterborne stain is applied. Significant improvements in waterborne wood stains are disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 07/951,619, filed on Sep. 28, 1992, by R. E. Dean and E. E. McEntire, wherein a vinyl latex resin is disclosed as providing desirable barrier properties in a stain composition.
Stains that are to be used in finer quality woodwork, such as high quality furniture, have additional requirements. More specifically, stains that are hand-rubbed require a certain amount of lubricity and a substantial "open time," that is, a delayed period of time before drying during which they remain workable by rubbing and wiping. It is known that the drying rate can be retarded by adding an oil to the composition, but the oil can have a negative effect on adhesion of subsequently applied coatings. It is also known, as disclosed in U.S. Pat. No. 5,149,729, to employ thickeners for various purposes. Combinations of thickeners have also been suggested in the prior art, but thickeners or combinations thereof had not heretofore been found that imparted to waterborne stains the same desirable wiping qualities possessed by oil based stains.
Thickeners available in the prior art include polysaccharides such as cellulosic materials (e.g., methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, and ethyl hydroxyethyl cellulose) and gums (e.g., xanthan gum and rhamsan gum). These cellulosic thickeners are sometimes available in the form of salts, e.g., sodium salts Clays are also commonly used as thickeners, and hectorite clay in particular has been suggested as a thickener in paints. Yet another type of thickener is known as "associative thickeners" which are synthetic polymeric materials that work in combination with surfactants or other polymers to increase viscosity. One type of associative thickeners are the alkali swellable acrylates such as those sold under the "Acrysol" name by Rohm & Haas. Another type of associative thickener are modified polyurethanes (hydrophobe-modified ethoxylated urethanes) as disclosed in "Interactions of HEUR Associative Thickeners with Waterborne Polyurethanes" by J. P. Kaczmarski et al. in Journal of Coatings Technology, Vol. 65, No. 818, March 1993. These HEUR thickeners associate with the hydrophobic portion of a polymer. A combination of associative thickener and cellulosic thickener in paints is discussed in the above-referenced Kaczmarksi et al. article. U.S. Pat. No. 5,149,729 discloses the combination of an associative thickener and a gum in a waterborne stain composition. A combination of hectorite clay and cellulosic thickeners in paint is disclosed by P. Jenness in "Rheological Design Using Laponite and Cellulose Ethers" in Pitture e Vernici Europe, 6:91.
It should be noted that the considerations for thickener selection for paints and for stains are not the same. Leveling and flow are usually primary considerations with paints, but are of little concern with stains. Stains, on the other hand, require lubricity and working time, factors which are unlikely to be considered when formulating paints.